1. Field of the Invention
The present invention relates to a method for detecting the locations of underground electrically conductive bodies in accordance with the preamble of claim 1, said method being particularly useful for prospecting deeply located parts of old mining districts. The invention also relates to apparatus for carrying out the method.
2. Background Information
The last 10 to 15 years have shown an increased interest in the development of a geophysical prospecting technique for detecting the presence of deeply-buried valuable deposits, as a complement to geophysical surface prospecting methods.
Of the underground geophysical prospecting methods proposed, the method which involves the use of electromagnetic borehole probes has found increasing interest. This technique is based on the general principle of passing an alternating current through a loop placed on the ground surface, normally a square loop having a side-size of from 100.times.100 m up to 1000.times.1000 m or larger, such as to generate a primary electromagnetic field which propagates underground. The electromagnetic field thus generated induces currents in each conductive body that is located within the propagation area of the primary field. These induced currents in turn generate a secondary electromagnetic field. Methods have been proposed for determining the time decay of an induced secondary field, subsequent to having discontinued the supply of direct current through the ground loop. These methods are referred to generally as "time-domain methods", of which SIROTEM (trademark) originating in Australia is one such method. When practicing these known methods, only that component of the induced secondary field which is parallel with the axis of the borehole at the place where said locations are determined is measured, and consequently no signals are obtained which disclose whether the field direction is perpendicular to the borehole axis or not. The use of a so-called horizontal-field induction coil has also been proposed, these coils being used two-and-two in each probe. One drawback with this methodology, however, is that it is necessary to establish a ground loop which is located symmetrically around the borehole where the locations of valuable deposits are to be determined.
The later development of methods based on electromagnetic alternating fields with sinusoidal time-dependent amplitude (frequency-domain methods) have further improved deep prospecting possibilities. A fundamental feature of these methods is that the field components are measured while maintaining the primary field, wherein the sum of the primary field and the secondary field are measured in different ways. U.S. Pat. No. 2,919,397 (L. W. Morley) teaches an inductive prospecting arrangement which includes a transmitter coil for generating a primary field, a receiver having three coils, an independent quadrating unit for each receiver coil, and a pickup-coil which is physically and electrically isolated from and located at a distance from the receiver coils. A primary field concerned with barren ground or a vacuums is subtracted by the pickup-coil which supplies information concerning the strength of the transmitter signals. Although the arrangement is described solely with regard to prospecting above ground level, e.g. is airborne, it is conceivable that the arrangement could also be used for prospecting underground in a borehole. In this latter case, however, the relatively unsophisticated technique described would probably be unable to subtract a primary field with the pickup coil. GB-B-2148012 (R. W. Cobcroft, AU) thus proposes the use of three linear independent field detecting coils for determining the total, combined, magnetic field in each of three directions, wherein the so-called polarization ellipse formed by vectors in the composite magnetic field is determined at each measuring location. The actual presence of a polarization ellipse indicates the presence of conductive bodies in the immediate vicinity, although the direction to and the distance from the indicated bodies can only be determined very approximately. GB-A 1 467 943 (Bureau de Recherches) teaches a borehole prospecting method based on the use of low-frequency radio waves. This method is based on the decrease in the amplitude of the magnetic field vector as the radio waves propagate down into the ground. The method employs a single-component probe which is directed along the borehole and which registers that component of the sum of the primary and secondary fields which is parallel with the coil, i.e. which is parallel with the borehole axis. The method assumes that the ground is homogenous and isotropic, i.e. has the same resistivity overall and the same properties in all directions. There is no description relating to the calculation of the primary and secondary fields, although the amplitude and phase position of the receiver probe signal are determined, the values obtained being later used to establish the resistivity of the surroundings.
In the 1980s, Boliden developed a borehole measuring method based on a similar technique, referred to as BHEM (borehole-EM), and a surface survey method (EM3). The BHEM-technique is described in more detail in the report entitled "Borehole Geophysics for Mining and Geotechnical Applications" published in Geological Survey of Canada Paper 85-27 (Toronto, 1983). Boliden's method also enables the position of the underground conductive bodies indicated by the method to be determined, at least in the case of valuable deposits located at reasonable depths beneath the ground surface.
The Boliden BHEM-technique is primarily based on the principle of placing a large cable loop on the surface of the ground and subsequently sending through the cable a sinusoidal alternating current having a frequency within the range of some few Hz to several Khz. As indicated in the aforegoing, this current regenerates a primary electromagnetic field which interacts with conductive bodies, particularly the magnetic component of the field which induces currents in the conductive bodies. These secondary currents generate a secondary magnetic field which is superimposed on the primary field. Receiver coils of a three-component system are placed and accurately orientated at several different positions on or above the ground surface (EM3) or are mounted inside a borehole probe and positioned in a borehole, beneath the surface of the ground. The magnetic component of the electromagnetic field comprising both the primary and the secondary electromagnetic fields, this combined field being referred to as the total electromagnetic field, is determined at each location in which a receiver coil system has been placed. Several frequencies can be transmitted, received and recorded simultaneously, or in close sequence. The amplitude and the phase of the received field is measured and recorded in a receiver unit, to which the receiver coils are connected.
Information concerning the primary field, i.e. the field generated by the cable loop, is transmitted to the receiver unit in the form of high-frequency radio waves, thereby enabling both the amplitude of the primary field and the phase difference between the primary field and the total field measured in each receiver coil to be determined. In this case, the borehole probe measures the EM-field parallel with the borehole and measures the horizontal field at right angles to the hole, and also measures a third component which is perpendicular to the other two components. The probe fitted with said three coils is provided with a cradle which is journalled at two points and which ensures that the two coils which do not measure the field component that lies parallel with the borehole will always be positioned correctly, because of the gravitational forces acting thereon. This is particularly important with regard to measuring accuracy. The three components form a right-orientated coordinate system. Data measured in this way is recorded and processed, by first calculating the primary field at each selected measuring location. A model based on a simplified theoretical basis, wherein both amplitude and phase are determined for each of the three components (X, Y, Z) corresponded by the three coils, has been developed for calculating the configuration or appearance of the primary field at different depths beneath the surface of the ground. This theoretical basis is determined for the known bedrock or base rock at the sampling site in the absence of conducting bodies in the vicinity. The primary field data is then calculated and recorded, whereafter this data can be subtracted from corresponding data for the total field, such as to obtain the secondary field data. This calculation is preferably effected by dividing each space (three-dimensional) component (X, Y, Z), the amplitude and phase of which are known, into two phase components, the real component, which is in phase with the vectors of the local primary field, and the imaginary component, which is 90 degrees out of phase with the vectors of the local primary field. When, as in the general case, the three space components in the local primary field do not have mutually the same phase position, the real component and the imaginary component of all three space components of the primary field will normally be separated from zero. Thus, the vectorial calculation involves subtracting the primary field from the measured field for each of the six components, there being obtained a residual field in six components, namely real and imaginary components for each of the X, Y and Z directions. This is equivalent to knowing the direction, amplitude and phase of the secondary field.
The manner in which these calculations are made is described and exemplified in more detail in the article "Borehole Geophysics for Mining and Geotechnical Applications" mentioned in the introduction.
Since the secondary field is therefore known, this knowledge can be used to calculate the distribution of those currents which generate the secondary field. These currents flow within those parts of the ground which are more conductive than the surroundings and which, under favourable conditions and more specifically under conditions of high conductivity contrasts, low general conductivity and a homogenous primary field, are concentrated at the edges of the superior conductors. The boundaries of a target for further prospective drilling, for the purpose of further investigating the possible presence of a valuable deposit, are determined in this way.
In addition to assuming mathematical models for calculating the magnetic field at any selected location in the homogenous bedrock or ground, the BHEM-method also assumes that it is possible to measure, determine and record instantaneously the phase of such a field as it exists in the proximity of the probe measuring process in progress. Since the function of the phase reference is particularly important for distinguishing the secondary field from the primary field, and also for defining the real and imaginary components of the secondary field, it is necessary to use a correctly indicating and stable phase reference. The phase reference, which according to the aforegoing is transmitted from the transmitter to the receiver in the form of radio waves, actually involves recording the precise time of the zero crossing of the frequency used. It is also theoretically possible, of course, to determine and record this time point with the aid of particularly accurate clocks with precise timing, each clock sending signals to the transmitters and receivers. In practice, however, this would require the use of clocks (atomic clocks) of such an advanced nature as to render the solution prohibitive for use with the BHEM-method.
Thus, at present, the BHEM-method can only be used when the transmission of signals from transmitter to receiver is not disturbed by intermediate rock formations, both in the horizontal and the vertical directions. If the transmitter/receiver are located underneath a conductive earth-covering, it is also possible that the desired anticipatability will be lost, which may also occur when an electrically conductive rock formation lies over the crystalline rock formation to be investigated. When prospecting in a borehole down a mine, it is often difficult to draw a cable from a surface-located radio receiver down through the borehole. The provision of such a cable is necessary, however, because the frequencies of radio waves are so high that the radio waves only penetrate a short distance down into the ground.